This invention relates to scanning acoustic microscopes used in the non-destructive testing of microcircuit parts, and is addressed primarily to a system and method for enhancing the capability of such systems to immobilize the parts under test.
A scanning acoustic microscope typically has an ultrasonic beam generator that is traversed rapidly back and forth over a part under test. To traverse the entire part, either the beam generator is scanned in two dimensions, or in one dimension, as the part is translated through the beam in the orthogonal dimension.
The image output of the scanning acoustic microscope is employed for the non-destructive analysis of the internal physical characteristics of the part. The scanning acoustic microscope is able to penetrate through the part surface and image microscopic internal features in solids such as metals, ceramics, polymers, and composites. Typical components tested include microelectronic components such as integrated circuits (IC's), multi-layer ceramic capacitors, and multi-chip modules. Faults typical of the parts tested include delaminations, cracks, tilts of discrete layers, disbonds, underfill coverage, and voiding.
Such components may be carried to the scanning station in trays known in the art as “JEDEC” trays. JEDEC trays are characterized by comprising an X-Y matrix of individual cells or pockets custom shaped and sized for the particular parts being carried.
It is a characteristic of high-frequency ultrasound that, while able to penetrate solids such as those described, high frequency ultrasound beams cannot pass through an air gap between the ultrasound beam generator and the part under test without severe attenuation. A fluid medium is therefore used to couple the high-frequency output of the scanning head of the ultrasonic beam generator to the part. The fluid medium is usually water, although alcohol and other fluids may be used. In one common approach, a coupling fluid is dispensed in a falling stream or an upwardly ejected fountain which surrounds the ultrasonic beam.
It is the inevitable design trend in microelectronics that parts such as IC's are becoming ever smaller. And as they diminish in size, the parts become more difficult to handle and manipulate, especially when tested in a production environment. In particular, a coupling fluid stream is very apt to agitate and even dislodge such small parts from the trays as they move through the scanning acoustic microscope.
It is important to understand that JEDEC trays were developed as a means to carry integrated circuits and other semiconductor products from one semiconductor fabrication step or station to another. As the individual parts may be removed from and placed in their individual pockets a number of times during the fabrication process, typically by vacuum “pickers”, they must be loosely held in their pockets so that they may be easily removed and replaced without damage to the parts.
JEDEC trays were not designed to hold parts during inspection by a scanning ultrasonic microscope which requires that the parts be completely immobilized during inspection. The loose fit of part to pocket facilitates the basic transport function of JEDEC trays, but creates significant problems when trays of parts are inspected by a scanning ultrasonic microscope. One major problem is the dislodgement of parts, particular small parts, from the trays. A second significant problem is to immobilize the parts as they are being ultrasonically scanned.
If the parts are not immobilized during the insonification operation, the coupling fluid stream agitates the parts, causing them to move in their respective pockets as they are being interrogated by the scanned ultrasound beam. Movement of the parts during inspection distorts the inspection signals developed, producing errors which may be serious enough to vitiate the entire inspection process. For example, if the inspection process is intended to identify very fine cracks in a semiconductor die, the signal distortions introduced by part movement during ultrasonic interrogation may introduce errors of such magnitude that such cracks cannot be reliably detected.
Another problem with scanning JEDEC trays of parts is that the typically plastic trays may be warped as a result of defective manufacture or prolonged use or abuse. A warped tray changes the relative elevation and planarity of the parts in the two-dimensional array of parts which may result in inspection errors.
U.S. Pat. No. 5,684,252 to Kessler et al., of common ownership herewith, addresses the dislodgement and immobilization problems, disclosing and claiming a tray-fed scanning acoustic microscope system in which trays of parts are each paired with an open mesh screen to hold the parts in the trays as they pass through the scanning station. The screens are removed from the trays after the scanning operation has been completed. This technique requires a large number of screens of various sizes and configurations to accommodate different tray sizes and configurations. The screens represent an added capital and maintenance expense, and their handling generates a labor cost and delay.
U.S. Pat. No. 6,357,136, also issued to the owner of the present application, teaches a solution to the problem of dislodged parts, and in some applications of part immobilization, by providing a single stationary hold-down structure between the ultrasound beam generator and the part-holding trays. The cost of multiple screens is overcome by the use of a single hold-down structure which prevents the coupling fluid from dislodging parts from their trays during the inspection process.
The system and method of the U.S. Pat. No. 6,357,136 functions best when used with JEDEC or other trays of parts in which height (thickness) of the individual parts (typically identical) being tested is greater than the depth of the pocket which holds the part. The stationary hold-down structure is thus able to physically engage the protruding parts and firmly immobilize them in their respective pockets during interrogation by the ultrasonic beam.
However, in applications in which the height of the inspected parts is less than the containing pocket depth, the stationary hold-down structure, while still effective to prevent part dislodgement, is not as effective in immobilizing the contained parts during inspection. To combat the inspection accuracy problem, it is necessary in certain applications to slow the scanning rate, however this reduces the inspection throughput rate.